LWD and MWD systems are generally known in the art to make downhole measurements while a borehole is being drilled. Such systems measure various parameters and characteristics of the formation, such as its resistivity and the natural gamma ray emissions from the formation. Typically, signals which are representative of these measurements made downhole are relayed to the surface with a mud pulse telemetry device that controls the mud flow, encoding information in pressure pulses inside the drill string. The pulses travel upward through the mud to the surface, where they are detected and decoded so that the downhole measurements are available for observation and interpretation at the surface substantially in real time. In addition, it has also been found useful to provide a downhole computer with sufficient memory for temporarily storing these measurements until such time that the drill string is removed from the borehole.
NMR measurements are based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field, they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter T1, known as the spin-lattice relaxation time. The spin-lattice relaxation time T1 describes the coupling of nuclear spins to energy-absorbing molecular motions like rotation, vibration and translation.
Another related and frequently used NMR parameter is the spin-spin relaxation time constant T2 (also known as transverse relaxation time), which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. The mechanisms for spin-spin relaxation time T2 include, in addition to those contributing to T1, the exchange of energy between spins.
The pioneers in NMR measurement technologies envisioned the relaxation time T1 as the primary measurement result because T1 carries only information about the liquid-solid surface relaxation and bulk-fluid relaxation. Unlike T2, T1 is neither affected by rock-internal magnetic field gradients nor by differences in fluid diffusivity. Moreover, instrument artifacts influence T1 measurements to a much lesser degree than T2 measurements.
Despite the theoretical understanding of the superiority of T1 measurements, the oil industry entered the era of modern pulsed NMR logging in the early 1990s with logging tools designed to measure T2. The reasons for the switch from T1 to T2 were mostly stemmed from hardware limitations at that time. Specifically, the construction of the T1 recovery curve requires data collected with multiple wait times that range from a few milliseconds to several seconds. Acquiring T1 data using tools that operated in single-frequency mode without effective pre-polarization was too time-consuming and not feasible. T2 measurements, on the other hand, were faster and contained information similar to T1 at low resonance frequencies. As a result, T2 CPMG measurements were chosen as the main mode of tool operation.
Current developments of LWD and MWD technologies have overcome the hardware limitations and made efficient T1 measurements a reality. In particular, U.S. Pat. Nos. 6,531,868, 6,242,913 and 6,051,973, all to Prammer, disclose LWD and MWD methods and devices for obtaining T1 measurement data concerning petrophysical properties of formations. The contents of these patents are incorporated herein in their entirety by reference. The devices described in these patents contain two distinct operating modes, one designed for while-drilling operations and the other for wiping trips. Typically, the devices engage the motion-tolerant T1 mode when drilling motion is detected and switches over to T2 mode once drilling ceases. It is also very easy to acquire both T1 and T2 data over the same depth interval with an occasional wiping trip. This makes it possible to compare T1 and T2 data to improve the analysis of petrophysical properties. Prammer et al. in “A new direction in Wireline and LWD NMR”, presented at the 43rd Annual Logging Symposium Transactions: Society of Professional Well Log Analysis, 2002, presented several LWD and MWD applications where T1 and T2 data were gathered and compared. Contents of the Prammer et al. paper are incorporated herein by reference. It demonstrated that T1 measurements yields equivalent, and in some cased superior, formation evaluation answers. The contents of this article is also incorporated herein in its entirety by reference.
As demonstrated in Prammer et al. the relative insensitivity of T1 measurement with respect to field strength and field gradients gives it a clear advantage over T2 measurement in analyzing carbonate formations containing both micro and macro porosity systems. Current tools employing NMR T2 logging may not be able to distinguish the micro-pores from the macro-pores in the diffusivity analysis of such formations due to a phenomenon called “Diffusive Coupling”. The main cause of diffusive coupling is the weak surface relaxation of carbonates. Weak surface relaxation causes longer T2 times, a well-known NMR property of carbonate rocks. Given weak relaxation, a proton “lives longer”. Hence, if a proton originates in a small pore and stays alive for a long time, it may diffuse through the pore system, enter a larger pore, and eventually “die” in the larger pore. The signal from such a proton therefore does not reflect the size of the pore where it originates. Obviously, the reverse can also happen: a proton initially residing in a large pore may eventually “die” in a small one.
The overall effect of diffusive coupling is a blurred T2 distribution. Instead of a bi-modal T2 distribution, where the shorter T2 times are associated with the micro pores, and the longer T2 times with the larger pores, one generally observes a single large broad peak that sweeps mostly mid T2 ranges. Since the pore size information is lost, estimation of Swirr (irreducible water saturation) becomes problematic. This problem also affects the quality of the permeability estimates.
Current technologies attempt to overcome this problem in two ways. The first approach is to study the problem in the lab and trying to estimate the proper T2 “cutoffs” using laboratory NMR, then apply it to T2 logs. The second approach is to make certain assumptions in the pore system and try to deal with the problem through signal processing. (See Ramakrishnan et al. “Forward Models for Nuclear Magnetic Resonance in Carbonate Rocks”, paper SS, in 39th Annual Logging Symposium Transactions: Society of Professional Well Log Analysts, 2002, for a detailed description on current technologies for detecting diffusive coupling. The contents of the Ramakrishan et al. paper is incorporated herein in its entirety by reference.) Both approaches, unfortunately, requires making assumptions on the T2 measurement data and are generally not robust. Although the overall effects of diffusive coupling has been less pronounced because diffusive coupling is less of a problem in partially saturated rocks, the problem of NMR T2 logging in detecting diffusive coupling has become a factor that leads to the perception that NMR does not work in analyzing carbonate formations.
Accordingly, it is perceived that there is a need for a LWD and MWD-based system and method to detect diffusive coupling and distinguish different pore systems in the diffusivity analysis of carbonate formations.